Abstract: A mold assembly for manufacturing composite plastic products includes a mold cavity part and a lid, which is joined to form a cavity for foam precursors. The precursors expand in the cavity in a known manner to fill the cavity with a suitable layer of foamed material, such as foamed urethane. The mold cavity supports an exterior shell and the lid supports an insert for reinforcing a finished composite product. A sleeve is integrally formed on the preformed shell and it in turn is located within an access port formed through wall of the cavity part to define a slideway for receipt of a pour head extension. The extension is selectively clamped and unclamped with respect to the shell sleeve by an inflatable seal clamp which is formed as an annular ring with a pressurizable passage connected to a source of selectively applied pressure.